Light quantum entanglement source device based on lithium niobate film and preparation method of light quantum entanglement source device

Abstract

The invention relates to a light quantum entanglement source device based on a lithium niobate film and a preparation method of the light quantum entanglement source device. Comprising a substrate, and a silicon dioxide buffer layer and a double-layer lithium niobate film structure are sequentially arranged above the substrate; the spontaneous polarization directions of the upper-layer lithium niobate film and the lower-layer lithium niobate film in the double-layer lithium niobate film are opposite, and the upper-layer lithium niobate film is of a ridge waveguide structure; the ridge waveguide structure is provided with three functional areas from a horizontal angle, namely a frequency doubling area, a filter and a lower conversion area in sequence; a depletion layer is arranged above the filter, and conical couplers are arranged on the two sides of the frequency multiplication region and the lower conversion region. According to the light quantum entanglement source device provided by the invention, the fundamental frequency light band only generates loss of about 7500 dB / cm, and the loss of the frequency multiplication band is only about 400 dB / cm, so that the photon entanglement generation performance of the device is greatly improved. And the mode overlapping factor between the frequency doubling light and the fundamental frequency light is enhanced, the high-efficiency second-order nonlinear frequency doubling process is realized, and the normalization efficiency is in the magnitude of 103-104% / W / cm < 2 >.

Description

Technical Field

[0001] The present invention relates to an optical quantum entanglement source device based on a lithium niobate thin film and a preparation method thereof, belonging to the technical field of optical quantum devices. Background Art

[0002] With the rapid development of quantum information technology, as a core component in quantum communication, quantum computing, etc., the stability and efficiency of the performance of an optical quantum entanglement source have become the focus of research. Quantum entanglement is a phenomenon in quantum mechanics, which describes a special relationship between two or more particles, such that even if these particles are far apart in space, their states will immediately affect each other. An on-chip quantum entanglement source generates such an entangled state on a chip by using specific physical processes (such as second-order spontaneous parametric down-conversion, spontaneous four-wave mixing process).

[0003] The lithium niobate thin film is an important optoelectronic material. The nonlinear optical coefficient of lithium niobate is usually between 10 pm / V and 30 pm / V, which is much higher than that of many other common optical materials, making it have a strong nonlinear effect and being suitable for generating an efficient quantum entanglement source. In addition, the lithium niobate thin film also has excellent optoelectronic conversion efficiency (exceeding 80%), enabling high-performance optoelectronic devices. The lithium niobate thin film can maintain good optical performance in a wide temperature range (usually from room temperature to several hundred degrees Celsius) and environmental conditions. The lithium niobate thin film has a wide light transmission range (350 nm to 5200 nm), being suitable for visible light and near-infrared optical communication and optoelectronic applications. The refractive index difference between the lithium niobate thin film and the silica substrate is large (about ~0.7), and such a structure is particularly helpful for realizing a micro-nano optical structure with a compact structure and strong light wave confinement ability, thereby increasing the optical power density and integration density of the device and realizing the miniaturization and integration of the device.

[0004] Traditional optical quantum entanglement source preparation technologies have problems such as high preparation cost and poor stability, and it is difficult to meet the growing quantum technology requirements. There is a spontaneous four-wave mixing process in a silicon waveguide, which can be used to prepare an optical quantum entanglement source, but the silicon waveguide has the disadvantages of high loss and low coupling efficiency. The proton-exchanged and titanium-diffused lithium niobate optical waveguide can generate an optical quantum entanglement source by using its own second-order nonlinear spontaneous parametric down-conversion process, but the volume of this waveguide is large and the light field confinement ability is weak.

[0005] Chinese Patent Application CN109976066A discloses a non-degenerate polarization entanglement source system using a periodically poled lithium niobate thin film waveguide and its working method, including: a laser, a first half-wave plate, a dichroic mirror, a first detector, a second detector, a polarization beam splitter, a second half-wave plate, a first reflector, a second reflector, a periodically poled lithium niobate waveguide, etc. This invention uses a periodically poled crystal to obtain an entanglement source structure. However, in the loading method of this patent application, 532 nm is used as the pump light to generate entangled light sources at 810 nm and 1550 nm. The period of the required periodically poled grating is on the micron scale, and improper process control is likely to cause significant waveguide performance loss.

[0006] Chinese Patent Application CN116125726A discloses a design and preparation method of an on-chip entanglement source based on an X-cut periodically poled lithium niobate thin film. This patent application uses an X-cut lithium niobate thin film ridge waveguide as the waveguide structure and uses the periodic polarization method to fabricate two sets of electrodes with different periods in the propagation direction. The first set of electrode regions serves as the region for Type-I down-conversion, and the second set of electrode regions serves as the region for Type-II down-conversion to generate entangled light sources at 810 nm and 1550 nm. However, the process of this patent application is complex, and it is very difficult to simultaneously generate high-quality periodically poled structures with two different periods on a lithium niobate chip. Improper process control is likely to cause device performance degradation. Summary of the Invention

[0007] In view of the deficiencies of the prior art, the present invention provides an optical quantum entanglement source device based on a lithium niobate thin film and a preparation method thereof.

[0008] The present invention adopts the following technical solutions:

[0009] An optical quantum entanglement source device based on a lithium niobate thin film, including a substrate, and a silicon dioxide buffer layer and a double-layer lithium niobate thin film structure are sequentially arranged above the substrate; the spontaneous polarization directions of the upper lithium niobate thin film and the lower lithium niobate thin film in the double-layer lithium niobate thin film are opposite, and the upper lithium niobate thin film has a ridge waveguide structure; the ridge waveguide structure is provided with 3 functional regions from a horizontal angle, which are a frequency doubling region, a filter, and a down-conversion region in sequence; a depletion layer is arranged above the filter, and tapered couplers are arranged on both sides of the frequency doubling region and the down-conversion region.

[0010] Preferably according to the present invention, the substrate is a silicon substrate.

[0011] Preferably according to the present invention, the thickness of the double-layer lithium niobate thin film is 400 - 700 nm, the length is 0.5 - 2.5 cm, and the width is 0.2 - 1.2 cm; the thickness of the upper lithium niobate thin film is 200 - 400 nm, and the thickness of the lower lithium niobate thin film is 200 - 400 nm.

[0012] Further preferably, the double-layer lithium niobate thin film has a thickness of 600 nm, a length of 1 cm, and a width of 0.7 cm; the upper-layer lithium niobate thin film has a thickness of 280 nm, and the lower-layer lithium niobate thin film has a thickness of 320 nm. These parameters are obtained by using Python, Lumerical or Comsol to perform mathematical modeling on the geometric structure of the optical quantum entanglement source device, and then using a differential solver to solve the geometric structure obtained from the mathematical modeling to determine the optimal geometric conditions.

[0013] Preferably according to the present invention, the tangential direction of the double-layer lithium niobate thin film is x-cut or z-cut.

[0014] Preferably according to the present invention, the ridge waveguide has a width of 1100 - 1400 nm and a thickness of 400 - 500 nm.

[0015] Further preferably, the ridge waveguide has a width of 1200 nm and a thickness of 450 nm. These parameters are obtained by using Python, Lumerical or Comsol to perform mathematical modeling on the geometric structure of the optical quantum entanglement source device, and then using a differential solver to solve the geometric structure obtained from the mathematical modeling to determine the optimal geometric conditions.

[0016] Preferably according to the present invention, the depletion layer is made of gold, has a thickness of 15 - 25 nm, a length of 45 - 55 μm, and a width of 4 - 6 μm; the size of the depletion layer is less than or equal to that of the filter.

[0017] Further preferably, the depletion layer has a thickness of 20 nm, a length of 50 μm, and a width of 5 μm. These parameters are obtained by using Python, Lumerical or Comsol to perform mathematical modeling on the geometric structure of the optical quantum entanglement source device, and then using a differential solver to solve the geometric structure obtained from the mathematical modeling to determine the optimal geometric conditions.

[0018] Preferably according to the present invention, the length of the tapered coupler is 95 - 105 μm; an oxygen-silicon protective layer is further provided on the surface of the tapered coupler, and the thickness of the oxygen-silicon protective layer is 3 - 5 μm.

[0019] Further preferably, the length of the tapered coupler is 100 μm, and the thickness of the oxygen-silicon protective layer is 4 μm.

[0020] Further preferably, lens optical fibers are further provided on both sides of the tapered coupler, and their function is to input light and output light.

[0021] The preparation method of the above-mentioned optical quantum entanglement source device based on lithium niobate thin film includes the following steps:

[0022] (1) Deposit a silicon dioxide buffer layer on a silicon substrate; then clean the double-layer lithium niobate thin film to remove large inorganic particles and organic contaminants on the surface, and then fix the double-layer lithium niobate thin film on the silicon dioxide buffer layer by a bonding method; the double-layer lithium niobate thin film includes an upper-layer lithium niobate thin film and a lower-layer lithium niobate thin film with opposite spontaneous polarization directions;

[0023] (2) Spin-coat a photoresist on the surface of the double-layer lithium niobate thin film and expose it to form an etching mask required for dry etching; then perform dry etching. Through dry etching, the upper-layer lithium niobate thin film forms a ridge waveguide structure, and then anneal it at 280 - 320 °C in a dry oxygen state for 1 - 2 h; the ridge waveguide structure is divided into 3 functional regions from a horizontal angle, which are a frequency doubling region, a filter, and a down-conversion region in sequence;

[0024] (3) Prepare a depletion layer on the surface of the frequency doubling region through a lithography alignment process;

[0025] (4) Fix the sample obtained in step (3) on a wafer, and then form tapered coupler structures at both ends of the filter and the down-conversion region through a lithography alignment process as input and output ends; then prepare an oxygen-silicon protective layer on the surface of the tapered coupler by PECVD method or sputtering method;

[0026] (5) Spin-coat a photoresist on the surface of the tapered coupler structure of the sample obtained in step (4) and expose it to form an etching mask, and then perform deep etching on the wafer. After deep etching, the coupling interface of the tapered coupler structure will be exposed on the side wall of the etched trench, and then through die separation, an optical fiber coupler is obtained;

[0027] (6) Perform insertion loss testing and frequency doubling characteristic testing on the optical fiber coupler, couple the two tapered coupler structures of the optical fiber coupler that meet the standards with lens fiber end faces and cure them with ultraviolet glue. The two lens fibers at both ends are used as the optical input port and the optical output port respectively to obtain an optical quantum entanglement source device based on lithium niobate thin film.

[0028] Preferably according to the present invention, in step (1), the cleaning and drying are as follows: rinse the double-layer lithium niobate thin film with deionized water to remove large inorganic particles; then perform ultrasonic cleaning with soapy water to remove organic contaminants and inorganic particles, and finally rinse with deionized water and dry with nitrogen.

[0029] Preferably according to the present invention, in step (2), the spin-coated photoresist is a spin-coated ultraviolet photoresist or an electron beam photoresist, and the exposure is electron beam exposure or ultraviolet step exposure.

[0030] Preferably according to the present invention, in step (3), the lithography alignment process is to first use ultraviolet lithography to form a depletion layer pattern, then use a coating method to form a depletion layer on the surface of the frequency doubling region, and finally use the lift-off method to remove the photoresist and the excess thin film.

[0031] Preferably according to the present invention, in step (4), in the overlay process, a tapered structure pattern is first formed at both ends of the filter and the down-conversion region by an electron beam lithography method, and then a tapered structure is formed at both ends of the filter and the down-conversion region by dry etching or focused ion beam etching.

[0032] Preferably according to the present invention, in step (5), the spin-coated photoresist is a spin-coated ultraviolet photoresist or an electron beam photoresist, and the exposure is electron beam exposure or ultraviolet step exposure; the deep etching and dicing are conventional standard steps in a wafer processing factory.

[0033] Preferably according to the present invention, in step (6), the standard for the insertion loss test is that the insertion loss in the optical communication C band is not higher than 3 dB; the standard for the frequency doubling characteristic test is that the frequency doubling conversion efficiency is not lower than 4000% / W / cm / cm.

[0034] The technical principle of the present invention:

[0035] In the optical quantum entanglement source device provided by the present invention, in the frequency doubling region, the fundamental frequency light coupled into the device is converted into second harmonic light through mode phase matching, and its frequency doubling process is expressed as:

[0036] λ→λ / 2(1)

[0037] In formula (1), λ represents the wavelength.

[0038] The theoretical normalized conversion efficiency of this frequency doubling process is:

[0039]

[0040] In formula (2): ε0 represents the vacuum node constant, c represents the speed of light in vacuum, n i represents the effective refractive indices of the fundamental frequency light and the second harmonic light (i = 1, 2), λ represents the wavelength, ζ represents the nonlinear mode overlap integral, d eff is the effective nonlinear coefficient of lithium niobate in the near-infrared band, and Aeff represents the effective cross-sectional area of the waveguide.

[0041] The nonlinear mode overlap integral (ζ) is specifically as follows:

[0042]

[0043] The optical quantum entanglement source device of the present invention is divided into two regions with ±z orientations in the thickness direction. p(x,z) is the polarization distribution on the cross-section of the ridge waveguide of the optical quantum entanglement source device, and its value is -1 in the -z region and 1 in the +z region. During the ridge waveguide mode phase matching process, the device converts the fundamental mode TE 00 mode (X-cut waveguide) or TM 00 (Z-cut waveguide) of the fundamental frequency light into TE of the second harmonic light01 , TM 01 mode.

[0044] The power conversion efficiency of this process is related to the normalized conversion efficiency of the device itself, the length of the second harmonic generation region, and the fundamental optical power, and is expressed as:

[0045]

[0046] Among them, PowerConversionEfficiency represents the power conversion efficiency, η represents the normalized conversion efficiency in Equation 2, P F is the fundamental optical power, and L SH is the length of the second harmonic generation region. After second harmonic generation, the fundamental optical and second harmonic light enter the depletion layer, which is formed by depositing a functional film layer with light absorption on the surface of the ridge waveguide. Since the evanescent field distributions of the fundamental optical and second harmonic light modes at the air-lithium niobate interface are different, the absorption loss of the fundamental optical in this layer is large, and the absorption loss of the second harmonic light is small. By using this phenomenon, most of the fundamental optical entering the depletion layer can be filtered out, and most of the second harmonic light is retained and enters the down-conversion region. The purpose of this is to isolate the second harmonic light generated by the fundamental optical in the down-conversion region from affecting the entangled photons as much as possible, thereby improving the spectral purity of the entangled photon pairs. The absorption of the depletion layer for the fundamental optical and second harmonic light is exponentially related to the transmission distance.

[0047] The second harmonic light entering the down-conversion region will generate spontaneous parametric down-conversion due to the second-order nonlinearity of the lithium niobate ridge waveguide. A second harmonic photon generates a pair of mutually entangled fundamental optical photon pairs, and the process can be expressed as:

[0048] λ / 2 → λ1 + λ2 (5)

[0049] And it satisfies:

[0050] 1 / λ1 + 1 / λ2 = 2 / λ (6)

[0051] In Equations (5) and (6), λ represents the original wavelength, and λ1 and λ2 represent the two new wavelengths generated in the spontaneous parametric down-conversion.

[0052] Spontaneous parametric down-conversion can be understood as a parametric amplification process of spontaneous emission. Each incident pump photon splits into two photons with lower energy with a certain probability, and the down-converted photons have characteristics such as time, polarization, frequency, and spin entanglement. Since spontaneous emission is a continuous spectrum, the spontaneous parametric down-conversion optical field has a wide spectral distribution from the pump frequency (2c / λ) to the lattice resonance frequency.

[0053] The beneficial effects of the present invention:

[0054] 1. The present invention provides a novel lithium niobate thin-film-based optical quantum entanglement source device with a double-layer lithium niobate thin-film structure. The spontaneous polarization directions of the upper and lower lithium niobate thin films in the double-layer lithium niobate thin films are opposite. Through etching, the upper lithium niobate thin film is formed into a ridge waveguide structure, and then the ridge waveguide structure is divided into three functional regions: a frequency doubling region, a filter, and a down-conversion region. And a depletion layer is arranged above the filter. Under this structure, the frequency doubling region converts the fundamental light coupled into the device into second-harmonic light through mode phase matching. After frequency doubling, the fundamental light and the second-harmonic light enter the depletion layer. Since the evanescent field distributions of the modes of the fundamental light and the second-harmonic light at the air-lithium niobate interface are different, the absorption loss of the fundamental light in this layer is relatively large, while the absorption loss of the second-harmonic light is relatively small. By using this phenomenon, most of the fundamental light entering the depletion layer can be filtered out, and most of the second-harmonic light is retained and enters the down-conversion region. According to the calculation and test of the inventors of the present application, by adopting the strategy of covering the depletion layer on the top and side, the loss in the fundamental light band is only about 7500 dB / cm, and the loss in the second-harmonic band is even only about 400 dB / cm, thus greatly improving the performance of the device in generating photon entanglement. And the double-layer lithium niobate thin-film structure greatly enhances the mode overlap factor between the second-harmonic light and the fundamental light, realizing a highly efficient second-order nonlinear frequency doubling process. The normalized efficiency of this device measured experimentally is in the range of 10 3 ~10 4 % / W / cm 2 order of magnitude.

[0055] 2. The present invention combines photolithography and dry etching to prepare the ridge waveguide structure, reducing the process complexity, optimizing the compatibility and flexibility of the preparation process. It can be completed only with a general electron beam exposure machine, an ultraviolet step-and-repeat lithography machine, and a dry etching machine, effectively overcoming the problems of high equipment cost, complex structure, and low production capacity in the preparation of waveguide structures by traditional laser direct writing methods and ion implantation methods, and being easy for large-scale production.

[0056] 3. The double-layer lithium niobate thin film used in the present invention is a commercial product. This double-layer structure is stable, and the nm-level uniformity and flatness can be achieved on the entire large-size wafer. Moreover, this structure does not require periodic polarization treatment processes that are incompatible with the CMOS process, so it can bring very stable second-order nonlinear performance and a more compatible preparation process flow.

[0057] 4. The optical quantum entanglement source device based on lithium niobate thin film provided by the present invention has the advantages of high performance, low transmission loss, and small size (in the micron range). Moreover, it is an optical waveguide chip that can well maintain the crystal nonlinearity, has a wide crystal nonlinear tuning range, good photon entanglement effect, and can generate broadband quantum entanglement photon pairs by only using a single-wavelength laser in the optical communication band, without the need for an additional expensive laser in the 765 - 785nm band for pumping. This not only reduces the complexity of the entire system but also reduces the cost of the entire optical quantum entanglement source system. More importantly, combining an on-chip integrated optical communication laser will make it possible to achieve a fully integrated optical quantum entanglement source. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 FIG. is a schematic cross-sectional structure diagram of the frequency doubling region and the down-conversion region in the optical quantum entanglement source device based on lithium niobate thin film of the present invention.

[0059] Figure 2 FIG. is a schematic cross-sectional structure diagram of the filter in the optical quantum entanglement source device based on lithium niobate thin film of the present invention.

[0060] Figure 3 FIG. is a schematic structure diagram of the ridge waveguide of the optical quantum entanglement source device based on lithium niobate thin film of the present invention.

[0061] Figure 4 FIG. shows the performance data of the optical quantum entanglement source device based on lithium niobate thin film of the present invention.

[0062] Figure 5 FIG. is a schematic working principle diagram of the optical quantum entanglement source device based on lithium niobate thin film of the present invention.

[0063] Figure 6 FIG. is a distribution diagram of the high-order mode of the fundamental frequency light and the second harmonic light of the optical quantum entanglement source device based on lithium niobate thin film of the present invention.

[0064] In the figure: 1: upper lithium niobate thin film; 2: lower lithium niobate thin film; 3: silicon dioxide buffer layer; 4: silicon substrate; 5: depletion layer; the arrow is the polarization direction of the lithium niobate thin film. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0065] To make the technical problems, technical solutions, and advantages to be solved by the present invention clearer, the following will be described in detail with reference to the accompanying drawings and specific embodiments, but not limited thereto. For those not elaborated in detail in the present invention, they are all in accordance with the conventional techniques in the art.

[0066] The double-layer lithium niobate thin film used in the present invention is available from Jinan Jingzheng Electronic Technology Co., Ltd.

[0067] Example 1

[0068] As Figures 1 to 3As shown in the figure, an optical quantum entanglement source device based on a lithium niobate thin film includes a silicon substrate, a silicon dioxide buffer layer and a double-layer lithium niobate thin film structure are sequentially arranged above the silicon substrate; the spontaneous polarization directions of the upper lithium niobate thin film and the lower lithium niobate thin film in the double-layer lithium niobate thin film are opposite, and the upper lithium niobate thin film has a ridge waveguide structure; the ridge waveguide structure is provided with 3 functional regions from a horizontal angle, which are a frequency doubling region, a filter and a down-conversion region in sequence; a depletion layer is arranged above the filter, and tapered couplers are arranged on both sides of the frequency doubling region and the down-conversion region.

[0069] The thickness of the double-layer lithium niobate thin film is 600 nm, the length is 1 cm, the width is 0.7 cm, the thickness of the upper lithium niobate thin film is 280 nm, and the thickness of the lower lithium niobate thin film is 320 nm. The width of the ridge waveguide is 1200 nm, and the thickness of the ridge waveguide is 450 nm. The material of the depletion layer is gold, the thickness is 20 nm, the length is 50 μm, and the width is 5 μm. The length of the tapered coupler is 100 μm. The length of the frequency doubling region is 4750 μm, the length of the filter is 50 μm, and the length of the down-conversion region is 5000 μm. These parameters are all obtained by using Python, Lumerical or Comsol to perform mathematical modeling on the geometric structure of the optical quantum entanglement source device, and then using a differential solver to solve the geometric structure obtained from the mathematical modeling to determine the optimal geometric conditions.

[0070] An oxygen-silicon protective layer is also arranged on the surface of the tapered coupler, and the thickness of the oxygen-silicon protective layer is 4 μm. Lens optical fibers are also arranged on both sides of the tapered coupler, and their functions are to input light and output light.

[0071] The preparation method of the above-mentioned optical quantum entanglement source device based on a lithium niobate thin film includes the following steps:

[0072] (1) Deposit a silicon dioxide buffer layer on a silicon substrate with a length of 2 cm and a width of 1 cm; then rinse the x-cut double-layer lithium niobate thin film with deionized water to remove large inorganic particles, then perform ultrasonic cleaning with soapy water to remove organic contamination and inorganic microparticles, and finally rinse with deionized water and dry with nitrogen; then fix the double-layer lithium niobate thin film on the silicon dioxide buffer layer by a bonding method; the double-layer lithium niobate thin film includes an upper lithium niobate thin film and a lower lithium niobate thin film with opposite spontaneous polarization directions;

[0073] (2) Spin-coat an electron beam photoresist on the surface of the double-layer lithium niobate thin film and perform electron beam exposure to form an etching mask required for dry etching; then perform dry etching. Through dry etching, the upper lithium niobate thin film forms a ridge waveguide structure, and then anneal the etched sample at 300 °C in a dry oxygen state for 1.5 h; the ridge waveguide structure is divided into 3 functional regions from a horizontal angle, which are a frequency doubling region, a filter and a down-conversion region in sequence;

[0074] (3) First, use ultraviolet lithography to form the depletion layer pattern, then use a coating method to form the depletion layer structure on the surface of the frequency doubling region, and finally use the lift-off method to remove the photoresist and the excess thin film to obtain the depletion layer;

[0075] (4) Fix the sample obtained in step (3) on the wafer. First, use electron beam lithography to form the tapered structure pattern at both ends of the filter and the down-conversion region, and then use dry etching to form the tapered structure at both ends of the filter and the down-conversion region as the input end and the output end; then, prepare an oxygen-silicon protective layer on the surface of the tapered coupler by sputtering to form a complete tapered coupler structure;

[0076] (5) Spin-coat an electron beam photoresist on the surface of the tapered coupler structure of the sample obtained in step (4) and perform electron beam exposure to form an etching mask, and then perform deep etching on the wafer. After deep etching, the coupling interface of the tapered coupler structure will be exposed on the sidewall of the etched groove, and then through dicing, an optical fiber coupler is obtained;

[0077] (6) Perform insertion loss testing and frequency doubling characteristic testing on the optical fiber coupler. Couple the two tapered coupler structures of the optical fiber coupler that meet the standards with lens optical fiber end faces and cure with ultraviolet glue. The two lens optical fibers at both ends are used as the optical input port and the optical output port respectively to obtain an optical quantum entanglement source device based on lithium niobate thin film;

[0078] The standard of the insertion loss test is that the insertion loss in the optical communication C band is not higher than 3 dB; the standard of the frequency doubling characteristic test is that the frequency doubling conversion efficiency is not lower than 4000% / W / cm / cm.

[0079] Example 2

[0080] An optical quantum entanglement source device based on lithium niobate thin film, comprising a silicon substrate, a silicon dioxide buffer layer and a double-layer lithium niobate thin film structure are sequentially arranged above the silicon substrate; the spontaneous polarization directions of the upper lithium niobate thin film and the lower lithium niobate thin film in the double-layer lithium niobate thin film are opposite, and the upper lithium niobate thin film has a ridge waveguide structure; the ridge waveguide structure is provided with 3 functional regions from a horizontal angle, which are sequentially a frequency doubling region, a filter and a down-conversion region; a depletion layer is arranged above the filter, and tapered couplers are arranged on both sides of the frequency doubling region and the down-conversion region.

[0081] The thickness of the double-layer lithium niobate thin film is 560 nm, the length is 1 cm, and the width is 0.7 cm. The thickness of the upper-layer lithium niobate thin film is 260 nm, and the thickness of the lower-layer lithium niobate thin film is 300 nm. The width of the ridge waveguide is 1250 nm, and the thickness of the ridge waveguide is 400 nm. The material of the depletion layer is gold, the thickness is 20 nm, the length is 50 μm, and the width is 4.5 μm. The length of the tapered coupler is 95 μm. The length of the second-harmonic generation region is 4750 μm, the length of the filter is 50 μm, and the length of the down-conversion region is 5000 μm. These parameters are obtained by using Python, Lumerical or Comsol to perform mathematical modeling on the geometric structure of the optical quantum entanglement source device, and then using a differential solver to solve the geometric structure obtained from the mathematical modeling to determine the optimal geometric conditions.

[0082] An oxygen-silicon protective layer is further provided on the surface of the tapered coupler, and the thickness of the oxygen-silicon protective layer is 3 μm. Lens fibers are also provided on both sides of the tapered coupler, and their function is to input light and output light.

[0083] The preparation method of the above-mentioned optical quantum entanglement source device based on lithium niobate thin film includes the following steps:

[0084] (1) Deposit a silicon dioxide buffer layer on a silicon substrate with a length of 2 cm and a width of 1 cm; then rinse the x-cut double-layer lithium niobate thin film with deionized water to remove large inorganic particles, then perform ultrasonic cleaning with soapy water to remove organic contaminants and inorganic particles, and finally rinse with deionized water and dry with nitrogen; then fix the double-layer lithium niobate thin film on the silicon dioxide buffer layer by a bonding method; the double-layer lithium niobate thin film includes an upper-layer lithium niobate thin film and a lower-layer lithium niobate thin film with opposite spontaneous polarization directions;

[0085] (2) Spin-coat a UV photoresist on the surface of the double-layer lithium niobate thin film and perform UV step exposure to form an etch mask required for dry etching; then perform dry etching. Through dry etching, the upper-layer lithium niobate thin film forms a ridge waveguide structure, and then anneal the etched sample at 300 °C in a dry oxygen state for 1.5 h; the ridge waveguide structure is divided into 3 functional regions from a horizontal angle, which are the second-harmonic generation region, the filter, and the down-conversion region in sequence;

[0086] (3) First, use the UV lithography method to form a depletion layer pattern, then use the coating method to form a depletion layer structure on the surface of the second-harmonic generation region, and finally use the lift-off method to remove the photoresist and the excess thin film to obtain the depletion layer;

[0087] (4) Fix the sample obtained in step (3) on the wafer. First, use electron beam lithography to form a tapered structure pattern at both ends of the filter and the down-conversion region, and then use dry etching to form tapered structures at both ends of the filter and the down-conversion region as the input and output ends. Then, prepare an oxygen-silicon protective layer on the surface of the tapered coupler by sputtering to form a complete tapered coupler structure;

[0088] (5) Spin-coat a UV photoresist on the surface of the tapered coupler structure of the sample obtained in step (4) and perform UV step exposure to form an etching mask. Then, perform deep etching on the wafer. After deep etching, the coupling interface of the tapered coupler structure will be exposed on the sidewall of the etched trench. After dicing, an optical fiber coupler is obtained;

[0089] (6) Perform insertion loss test and second harmonic generation (SHG) property test on the optical fiber coupler. Couple the two tapered coupler structures of the optical fiber coupler that meet the standard with lens fiber end faces and cure with UV glue. The two lens fibers at both ends are used as the optical input port and the optical output port respectively to obtain an optical quantum entanglement source device based on lithium niobate thin film;

[0090] The standard for the insertion loss test is that the insertion loss in the optical communication C band is not higher than 3 dB; the standard for the SHG property test is that the SHG conversion efficiency is not lower than 4000% / W / cm / cm.

[0091] Example 3

[0092] An optical quantum entanglement source device based on lithium niobate thin film has a structure and preparation method as described in Example 2, except that the tangential direction of the double-layer lithium niobate thin film is z-cut.

[0093] Experimental Example

[0094] Test the relevant performance of the optical quantum entanglement source device based on lithium niobate thin film in Example 1, and obtain the following performance data. The results are as Figure 4 shown.

[0095] The specific test method is as follows: As Figure 5 shown, couple near-infrared laser into the optical quantum entanglement source device based on lithium niobate thin film in Example 1, keep the coupling efficiency stable and scan the wavelength, and record the second harmonic signal and the fundamental frequency optical signal at the same time. After the test, process the data to obtain the normalized conversion efficiency. The high-order mode distribution diagrams of the fundamental frequency light and the second harmonic light of the waveguide quantum entanglement source device in Example 1 are as Figure 6 shown.

[0096] It can be seen from Figure 4 that when the fundamental frequency light pump power is 0.4 mW, the Coincidence rate of photons is 3×10 5 per second. The brightness of the entangled photons is 8.3×10 5Hz / nm. In Example 1 of the present invention, the entanglement photon yield of the optical quantum entanglement source device based on lithium niobate thin film gradually increases with the pump light power, while the CAR value (Coincidence to accidental ration) decreases with the pump light power.

[0097] It can be seen from Figure 6 that the fundamental frequency light effectively generates the second harmonic light in the second harmonic generation region and transmits it to the filter region.

Claims

1. An optical quantum entanglement source device based on lithium niobate thin film, characterized in that It includes a substrate, on which there are successively a silicon dioxide buffer layer and a double-layer lithium niobate thin film structure; in the double-layer lithium niobate thin film, the spontaneous polarization directions of the upper lithium niobate thin film and the lower lithium niobate thin film are opposite, and the upper lithium niobate thin film has a ridge waveguide structure; the ridge waveguide structure has 3 functional regions from a horizontal angle, which are successively a frequency doubling region, a filter, and a down-conversion region; a depletion layer is arranged above the filter, and tapered couplers are arranged on both sides of the frequency doubling region and the down-conversion region.

2. The optical quantum entanglement source device based on a lithium niobate thin film according to claim 1, wherein The thickness of the double-layer lithium niobate thin film is 400 - 700 nm, the length is 0.5 - 2.5 cm, and the width is 0.2 - 1.2 cm; the thickness of the upper lithium niobate thin film is 200 - 400 nm, and the thickness of the lower lithium niobate thin film is 200 - 400 nm; Further preferably, the thickness of the double-layer lithium niobate thin film is 600 nm, the length is 1 cm, and the width is 0.7 cm; the thickness of the upper lithium niobate thin film is 280 nm, and the thickness of the lower lithium niobate thin film is 320 nm.

3. The optical quantum entanglement source device based on a lithium niobate thin film according to claim 1, wherein The tangential direction of the double-layer lithium niobate thin film is x-cut or z-cut; the width of the ridge waveguide is 1100 - 1400 nm, and the thickness of the ridge waveguide is 400 - 500 nm; Further preferably, the width of the ridge waveguide is 1200 nm, and the thickness of the ridge waveguide is 450 nm.

4. The optical quantum entanglement source device based on lithium niobate thin film according to claim 1, wherein The material of the depletion layer is gold, the thickness is 15 - 25 nm, the length is 45 - 55 μm, and the width is 4 - 6 μm; the size of the depletion layer is less than or equal to that of the filter; Further preferably, the thickness of the depletion layer is 20 nm, the length is 50 μm, and the width is 5 μm.

5. The optical quantum entanglement source device based on a lithium niobate thin film according to claim 1, characterized in that The length of the tapered coupler is 95 - 105 μm; an oxygen-silicon protective layer is also arranged on the surface of the tapered coupler, and the thickness of the oxygen-silicon protective layer is 3 - 5 μm; Further preferably, the length of the tapered coupler is 100 μm, and the thickness of the oxygen-silicon protective layer is 4 μm.

6. The optical quantum entanglement source device based on a lithium niobate thin film as described in claim 1, wherein lens optical fibers are also arranged on both sides of the tapered coupler, and their function is to input light and output light.

7. The preparation method of the lithium niobate thin film-based optical quantum entanglement source device according to any one of claims 1 to 6, characterized in that, It includes the following steps: (1) Deposit a silicon dioxide buffer layer on a silicon substrate; then clean the double-layer lithium niobate thin film to remove large inorganic particles and organic contaminants on the surface, and then fix the double-layer lithium niobate thin film on the silicon dioxide buffer layer by a bonding method; the double-layer lithium niobate thin film includes an upper lithium niobate thin film and a lower lithium niobate thin film with opposite spontaneous polarization directions; (2) Spin-coat and expose photoresist on the surface of the double-layer lithium niobate thin film to form an etching mask required for dry etching; then perform dry etching. Through dry etching, the upper lithium niobate thin film forms a ridge waveguide structure, and then anneal at 280 - 320 °C in a dry oxygen state for 1 - 2 h; the ridge waveguide structure is divided into 3 functional regions from a horizontal angle, which are successively a frequency doubling region, a filter, and a down-conversion region; (3) Prepare a depletion layer on the surface of the frequency doubling region through a nested etching process; (4) Fix the sample obtained in step (3) on the wafer, and then form a tapered coupler structure at both ends of the filter and the down-conversion region through a lithography process as the input end and the output end; then prepare an oxygen-silicon protective layer on the surface of the tapered coupler by PECVD method or sputtering method; (5) Spin-coat a photoresist on the surface of the tapered coupler structure of the sample obtained in step (4) and expose it to form an etching mask, and then perform deep etching on the wafer. After deep etching, the coupling interface of the tapered coupler structure will be exposed on the sidewall of the etched groove. After dicing, an optical fiber coupler is obtained; (6) Perform insertion loss test and second harmonic generation (SHG) property test on the optical fiber coupler. Couple the two tapered coupler structures of the optical fiber coupler that meet the standards with the end faces of the lens fibers and cure them with ultraviolet glue. The two lens fibers at both ends are used as the optical input port and the optical output port respectively to obtain an optical quantum entanglement source device based on lithium niobate thin film.

8. The preparation method according to claim 7, characterized in that, In step (1), the cleaning and drying are as follows: rinse the double-layer lithium niobate thin film with deionized water to remove large inorganic particles; then perform ultrasonic cleaning with soapy water to remove organic contamination and inorganic particles, and finally rinse with deionized water and dry with nitrogen; In step (2), the spin-coating of the photoresist is spin-coating of ultraviolet photoresist or electron beam photoresist, and the exposure is electron beam exposure or ultraviolet step exposure.

9. The preparation method according to claim 7, wherein, In step (3), the lithography process is to first form a depletion layer pattern by ultraviolet lithography method, then form a depletion layer on the surface of the second harmonic generation region by a coating method, and finally use the lift-off method to remove the photoresist and the excess thin film; In step (4), the lithography process is to first form a tapered structure pattern at both ends of the filter and the down-conversion region by electron beam exposure method, and then form a tapered structure at both ends of the filter and the down-conversion region by dry etching or focused ion beam etching.

10. The preparation method according to claim 7, characterized in that, In step (6), the standard of the insertion loss test is that the insertion loss in the optical communication C band is not higher than 3 dB; the standard of the second harmonic generation (SHG) property test is that the second harmonic conversion efficiency is not lower than 4000% / W / cm / cm.

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